use strict;

my @array = ("1.found","2.found", "4.found", "5.found");
my @array2;

my $i = 1;
foreach my $a(@array) {
    my ($n) = split /\./, $a;
    while ($n > $i) {
        push @array2, "$i.lost";
        $i++;
    }
    push @array2, $a;
    $i++;
}